1. Field of the Invention
The present invention relates to a method and an apparatus for depositing biocompatible material onto a substrate, and to articles formed thereby. More particularly, the present invention provides a method and an apparatus for using a laser to deposit thin films of biocompatible material of controlled chemical composition and crystalline structure onto a substrate to provide articles which may be used as medical, dental or orthopedic implants, other medical devices designed for short- or long-term contact with human or animal tissue, or for other applications in which a biocompatible material is required in thin film form.
2. Description of the Prior Art
Medical implant devices are being increasingly used for a variety of purposes. Because living organisms generally reject any foreign matter, such devices should appear to be of natural or nonforeign material.
The design of prosthetics or other devices to be implanted in the human body must take into consideration mechanical integrity as well as biocompatibility. A number of biocompatible ceramic compositions exist, but all lack sufficient mechanical strength for use in load-bearing applications.
It has been proposed to solve this materials problem by coating a bulk material, chosen for its mechanical strength and corrosion resistance, with a thin film of biocompatible material. Such a biocompatible coating may be inert, in which case a fibrous capsule of tissue will surround the implant over time, or it may be porous, so as to encourage the ingrowth of tissue into the pores. Alternatively, such a biocompatible coating may be resorbable, i.e., it may be designed to dissolve over time and be replaced by the natural host tissue. A fourth category of biocompatible coatings includes bioactive materials that elicit a specific biological response at the interface of the material which results in the formation of a bond between the tissues and the material.
An example of a biocompatible material is hydroxylapatite, i.e., Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2, which is the primary chemical constituent of bone. Various attempts have been made to deposit hydroxylapatite films onto metal substrates, the hydroxylapatite acting as a resorbable biocompatible coating. Amorphous hydroxylapatite and calcium phosphate materials, e.g., .alpha.-tricalcium phosphate, .beta.-tricalcium phosphate and tetracalcium phosphate are more resorbable than crystalline hydroxylapatite (HA).
Since the resorbability of a deposited film is a function of its local chemistry, as well as its crystal structure, a useful deposition technique for coating substrates with hydroxylapatite must allow for control of chemistry, crystallinity and morphology. Prior attempts to deposit hydroxylapatite films onto substrates have suffered from various drawbacks and limitations, as discussed below.
Plasma spraying has been used to coat both metal and ceramic substrates with hydroxylapatite films. (See Cook et al., J. Dental Research, vol. 65 (1986) p. 222.) A number of problems have been reported to result from the plasma-spraying process, however. Adhesion of the film to the substrates is poor, giving rise to failure of finished devices. Furthermore, plasma-spraying is a high temperature technique which is unsuitable for depositing films on substrates which may degrade at elevated temperatures. In addition, the high temperature nature of the plasma-spraying technique also tends to drive structural water from hydroxylapatite, leaving films comprising a small fraction of hydroxylapatite in a matrix of dehydrated hydroxylapatite, i.e., tricalcium phosphate (TCP).
Electrophoretic deposition has been used to deposit hydroxylapatite onto Ti-6Al-4V substrates. (See Ducheyne et al., Biomaterials, vol. 7 (1986) p. 97.) Adhesion of the film to the substrate is poor, and sintering at high temperatures is usually needed to provide satisfactory interfacial bonding. The high-temperature sintering tends to deplete the film of phosphorus due to the formation of Ti-P compound(s) at the substrate interface, and it tends to drive water from the hydroxylapatite.
Radio frequency magnetron sputtering deposition has been employed to deposit hydroxylapatite onto substrates. (See Ruckenstein et al., J. Colloid and Interface Science, vol. 96 (1983) p. 245.) However, sputtering deposition tends to drive water from the hydroxylapatite and sacrifices the crystallographic structure.
Ion Beam Sputtering has been employed to deposit hydroxylapatite on substrates, however, it generally yields films that are phosphorus-deficient and amorphous. (See Barthell et al., Materials Research Society Symposium Proceedings. vol. 110, (1989), p. 709 and Stevenson et al., ibid., p. 715.)